The self-consistent phonon theory of anharmonic lattice dynamics, devised independently by several authors using varying techniques and implemented computationally by Koehler, is here applied to the crystals of neon and argon. A Lennard-Jones 6-12 interatomic potential is assumed. The quantities calculated are the phonon spectrum and the bulk thermodynamic properties of thermal expansion, compressibility, and specific heat, all as a function of temperature at zero pressure. Although the computations are intended primarily to explore in detail the content of the self-consistent phonon approximation preparatory to incorporating the more elaborate expressions of the next higher approximation, comparison is made with the existing experimental data.

The development of an angle-dependent interaction potential for slightly nonspherical molecules is outlined, concentrating on molecular nitrogen. The suggested N2–N2 pair potential combines the radial dependence of the Barker type potential used for the rare gases with a modification of the treatment of anisotropy by the Berne–Pechukas Gaussian overlap model. This potential includes quadrupolar and long range dispersion contributions as determined from experiment or semiempirical studies. The valence potential parameters are determined by a fit of beam scattering and solid state measurements. This potential provides satisfactory agreement with Raman and neutron scattering measurements only when the anharmonic contributions to the lattice potential energy are included. A comparison of the suggested potential with a number of other presently available potentials, determined from a variety of measurements, as well as recommendations as to the limits of possible use are made. Possible steps which would lead to future improvements of the N2–N2 pair potential are outlined.

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